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Parallelization

Shared-memory parallelization with threads

Many compute-intensive loops in Trixi.jl are parallelized using the multi-threading support provided by Julia. You can recognize those loops by the @threaded macro prefixed to them, e.g.,

@threaded for element in eachelement(dg, cache)
  ...
end

This will statically assign an equal iteration count to each available thread.

To use multi-threading, you need to tell Julia at startup how many threads you want to use by either setting the environment variable JULIA_NUM_THREADS or by providing the -t/--threads command line argument. For example, to start Julia with four threads, start Julia with

julia --threads=4

If both the environment variable and the command line argument are specified at the same time, the latter takes precedence.

If you use time integration methods from OrdinaryDiffEq.jl and want to use multiple threads therein, you need to set the keyword argument thread=OrdinaryDiffEq.True() of the algorithms, as described in the [section on time integration methods](@ref time-integration).

!!! warning Not everything is parallelized yet and there are likely opportunities to improve scalability. Multi-threading isn't considered part of the public API of Trixi.jl yet.

Distributed computing with MPI

In addition to the shared memory parallelization with multi-threading, Trixi.jl supports distributed parallelism via MPI.jl, which leverages the Message Passing Interface (MPI). MPI.jl comes with its own MPI library binaries such that there is no need to install MPI yourself. However, it is also possible to instead use an existing MPI installation, which is recommended if you are running MPI programs on a cluster or supercomputer (see the MPI.jl docs to find out how to select the employed MPI library). Additional notes on how to use a system-provided MPI installation with Trixi.jl can be found in the following subsection.

!!! warning "Work in progress" MPI-based parallelization is work in progress and not finished yet. Nothing related to MPI is part of the official API of Trixi.jl yet.

[Using a system-provided MPI installation](@id parallel_system_MPI)

When using Trixi.jl with a system-provided MPI backend, the underlying p4est, t8code and HDF5 libraries need to be compiled with the same MPI installation. If you want to use p4est (via the P4estMesh) or t8code (via the T8codeMesh) from Trixi.jl, you also need to use system-provided p4est or t8code installations (for notes on how to install p4est and t8code see, e.g., here and here, use the configure option --enable-mpi). Otherwise, there will be warnings that no preference is set for P4est.jl and T8code.jl that can be ignored if you do not use these libraries from Trixi.jl. Note that t8code already comes with a p4est installation, so it suffices to install t8code. In order to use system-provided p4est and t8code installations, P4est.jl and T8code.jl need to be configured to use the custom installations. Follow the steps described here and here for the configuration. The paths that point to libp4est.so (and potentially to libsc.so) need to be the same for P4est.jl and T8code.jl. This could, e.g., be libp4est.so that usually can be found in lib/ or local/lib/ in the installation directory of t8code. The preferences for HDF5.jl always need to be set, even if you do not want to use HDF5 from Trixi.jl, see also issue #1079 in HDF5.jl. To set the preferences for HDF5.jl, follow the instructions described here.

In total, in your active Julia project you should have a LocalPreferences.toml file with sections [MPIPreferences], [T8code] (only needed if T8codeMesh is used), [P4est] (only needed if P4estMesh is used), and [HDF5] as well as an entry MPIPreferences in your Project.toml to use a custom MPI installation. A LocalPreferences.toml file created as described above might look something like the following:

[HDF5]
libhdf5 = "/usr/lib/x86_64-linux-gnu/hdf5/openmpi/libhdf5.so"
libhdf5_hl = "/usr/lib/x86_64-linux-gnu/hdf5/openmpi/libhdf5_hl.so"

[HDF5_jll]
libhdf5_hl_path = "/usr/lib/x86_64-linux-gnu/hdf5/openmpi/libhdf5_hl.so"
libhdf5_path = "/usr/lib/x86_64-linux-gnu/hdf5/openmpi/libhdf5.so"

[MPIPreferences]
__clear__ = ["preloads_env_switch"]
_format = "1.0"
abi = "OpenMPI"
binary = "system"
cclibs = []
libmpi = "/lib/x86_64-linux-gnu/libmpi.so"
mpiexec = "mpiexec"
preloads = []

[P4est]
libp4est = "/home/mschlott/hackathon/libtrixi/t8code/install/lib/libp4est.so"
libsc = "/home/mschlott/hackathon/libtrixi/t8code/install/lib/libsc.so"

[T8code]
libp4est = "/home/mschlott/hackathon/libtrixi/t8code/install/lib/libp4est.so"
libsc = "/home/mschlott/hackathon/libtrixi/t8code/install/lib/libsc.so"
libt8 = "/home/mschlott/hackathon/libtrixi/t8code/install/lib/libt8.so"

This file is created with the following sequence of commands:

julia> using MPIPreferences
julia> MPIPreferences.use_system_binary()

Restart the Julia REPL

julia> using P4est
julia> P4est.set_library_p4est!("/home/mschlott/hackathon/libtrixi/t8code/install/lib/libp4est.so")
julia> P4est.set_library_sc!("/home/mschlott/hackathon/libtrixi/t8code/install/lib/libsc.so")
julia> using T8code
julia> T8code.set_libraries_path!("/home/mschlott/hackathon/libtrixi/t8code/install/lib/")
julia> using HDF5
julia> HDF5.API.set_libraries!("/usr/lib/x86_64-linux-gnu/hdf5/openmpi/libhdf5.so", "/usr/lib/x86_64-linux-gnu/hdf5/openmpi/libhdf5_hl.so")

After the preferences are set, restart the Julia REPL again.

[Usage](@id parallel_usage)

To start Trixi.jl in parallel with MPI, there are three options:

  1. Run from the REPL with mpiexec(): You can start a parallel execution directly from the REPL by executing 
    julia> using MPI

julia> mpiexec() do cmd
         run(`$cmd -n 3 $(Base.julia_cmd()) --threads=1 --project=@. -e 'using Trixi; trixi_include(default_example())'`)
       end
    The parameter -n 3 specifies that Trixi.jl should run with three processes (or ranks in MPI parlance) and should be adapted to your available computing resources and problem size. The $(Base.julia_cmd()) argument ensures that Julia is executed in parallel with the same optimization level etc. as you used for the REPL; if this is unnecessary or undesired, you can also just use julia. Further, if you are not running Trixi.jl from a local clone but have installed it as a package, you need to omit the --project=@..
  2. Run from the command line with mpiexecjl: Alternatively, you can use the mpiexecjl script provided by MPI.jl, which allows you to start Trixi.jl in parallel directly from the command line. As a preparation, you need to install the script once by running 
    julia> using MPI

julia> MPI.install_mpiexecjl(destdir="/somewhere/in/your/PATH")
    Then, to execute Trixi.jl in parallel, execute the following command from your command line: 
    mpiexecjl -n 3 julia --threads=1 --project=@. -e 'using Trixi; trixi_include(default_example())'
  3. Run interactively with tmpi (Linux/MacOS only): If you are on a Linux/macOS system, you have a third option which lets you run Julia in parallel interactively from the REPL. This comes in handy especially during development, as in contrast to the first two options, it allows to reuse the compilation cache and thus facilitates much faster startup times after the first execution. It requires tmux and the OpenMPI library to be installed before, both of which are usually available through a package manager. Once you have installed both tools, you need to configure MPI.jl to use the OpenMPI for your system, which is explained here. Then, you can download and install the tmpi script by executing 
    curl https://raw.githubusercontent.com/Azrael3000/tmpi/master/tmpi -o /somewhere/in/your/PATH/tmpi
    Finally, you can start and control multiple Julia REPLs simultaneously by running 
    tmpi 3 julia --threads=1 --project=@.
    This will start Julia inside tmux three times and multiplexes all commands you enter in one REPL to all other REPLs (try for yourself to understand what it means). If you have no prior experience with tmux, handling the REPL this way feels slightly weird in the beginning. However, there is a lot of documentation for tmux available and once you get the hang of it, developing Trixi.jl in parallel becomes much smoother this way. Some helpful commands are the following. To close a single pane you can press Ctrl+b and then x followed by y to confirm. To quit the whole session you press Ctrl+b followed by :kill-session. Often you would like to scroll up. You can do that by pressing Ctrl+b and then [, which allows you to use the arrow keys to scroll up and down. To leave the scroll mode you press q. Switching between panes can be done by Ctrl+b followed by o. As of March 2022, newer versions of tmpi also support mpich, which is the default backend of MPI.jl (via MPICH_Jll.jl). To use this setup, you need to install mpiexecjl as described in the documentation of MPI.jl and make it available as mpirun, e.g., via a symlink of the form 
    ln -s ~/.julia/bin/mpiexecjl /somewhere/in/your/path/mpirun
    (assuming default installations).

!!! note "Hybrid parallelism" It is possible to combine MPI with shared memory parallelism via threads by starting Julia with more than one thread, e.g. by passing the command line argument julia --threads=2 instead of julia --threads=1 used in the examples above. In that case, you should make sure that your system supports the number of processes/threads that you try to start.

[Performance](@id parallel_performance)

For information on how to evaluate the parallel performance of Trixi.jl, please have a look at the [Performance metrics of the AnalysisCallback](@ref performance-metrics) section, specifically at the descriptions of the performance index (PID).

Using error-based step size control with MPI

If you use error-based step size control (see also the section on [error-based adaptive step sizes](@ref adaptive_step_sizes)) together with MPI you need to pass internalnorm=ode_norm and you should pass unstable_check=ode_unstable_check to OrdinaryDiffEq's solve, which are both included in ode_default_options.

Using parallel input and output

Trixi.jl allows parallel I/O using MPI by leveraging parallel HDF5.jl. On most systems, this is enabled by default. Additionally, you can also use a local installation of the HDF5 library (with MPI support). For this, you first need to use a system-provided MPI library, see also [here](@ref parallel_system_MPI) and you need to tell HDF5.jl to use this library. To do so with HDF5.jl v0.17 and newer, set the preferences libhdf5 and libhdf5_hl to the local paths of the libraries libhdf5 and libhdf5_hl, which can be done by

julia> using Preferences, UUIDs
julia> set_preferences!(
           UUID("f67ccb44-e63f-5c2f-98bd-6dc0ccc4ba2f"), # UUID of HDF5.jl
           "libhdf5" => "/path/to/your/libhdf5.so",
           "libhdf5_hl" => "/path/to/your/libhdf5_hl.so", force = true)

Alternatively, with HDF5.jl v0.17.1 or higher you can use

julia> using HDF5
julia> HDF5.API.set_libraries!("/path/to/your/libhdf5.so", "/path/to/your/libhdf5_hl.so")

For more information see also the documentation of HDF5.jl. In total, you should have a file called LocalPreferences.toml in the project directory that contains a section [MPIPreferences], a section [HDF5] with entries libhdf5 and libhdf5_hl, a section [P4est] with the entry libp4est as well as a section [T8code] with the entries libt8, libp4est and libsc. If you use HDF5.jl v0.16 or older, instead of setting the preferences for HDF5.jl, you need to set the environment variable JULIA_HDF5_PATH to the path, where the HDF5 binaries are located and then call ]build HDF5 from Julia.

If HDF5 is not MPI-enabled, Trixi.jl will fall back on a less efficient I/O mechanism. In that case, all disk I/O is performed only on rank zero and data is distributed to/gathered from the other ranks using regular MPI communication.